The invention relates to a die-welding coil for connecting tube bodies of composite foils having a metallic barrier layer and a plastic layer with a tube head by high-frequency induction welding, consisting of a cylindrical shaping die provided with an axial bore and of a hollow induction coil which may be connectable to a cooling agent source.
Ger. Pub. Sp. No. 26 28 014 discloses a die-welding coil having a die that serves as pressing tool and that is formed of an electrical non-conductor, such as a ceramic material. An induction coil cooperates with this pressing tool. Since electrical non-conductors, in general, and especially ceramics, are relatively poor heat conductors, the removal of heat from the welding zone in a short time is not possible, so that the number of welded connections which can be produced per unit of time is low. Moreover, the ceramic body heats up in time and effects a variation of the surface of the outer layers of the tube head shoulders; that is the synthetic plastics material loses its gloss and becomes undesirably matted. Furthermore, in the case of ceramic materials, there is always a danger of breakage on application of the necessary pressure in welding, since they are brittle, hard and difficult to work.
Ger. Pub. Sp. No. 20 23 652 discloses an induction coil consisting of a hollow conductor that can be cooled by means of a cooling agent conducted through the hollow conductor. According to this specification, the tubular body consists of thermally conductive aluminum and only its head attached at the end consists of non-conductive material in order to improve thermal conductivity away from the mandrel. This, however, only inappreciably improves the conducting of heat away from the welding zone. From this specification, it is also known to arrange the induction coil separately from the pressing die above the guide path of the mandrel and to move the tube parts to be welded with attached die through the induction field of the stationary coil.
The Ger. Pub. Sp. No. 26 28 014 discussed above also suggests arranging the induction coil in the mandrel and screening off the field in the tube, since then the possibility exists of producing the pressing die from metal in order to shorten the cooling time after the welding process. This arrangement, however, has a number of disadvantages since the coil and the requisite cooling and screening devices must be accommodated in the mandrel, where space is limited. Since the mandrel also must be movable in several directions, problems associated with transport of current and cooling agent are increased. Furthermore, only the marginal area of the coil can be exploited.
In the prior art arrangements, it is also not possible to heat specific parts more or less by specifically concentrated magnetic fields, since the coil opening is always larger than the tube head to be welded.